A variety of methods are known for dissipating heat in semiconductor devices. An existing method of heat dissipation employs a beryllium oxide (BeO) substrate which has a very high thermal conductivity. In addition, electrical currents may also be conducted by a refractory metallization and solder on the BeO substrate. Disadvantages of such systems include relatively high cost of manufacture, the toxic nature of BeO and relatively high electrical resistance of the refractory metallization. In fact, the use of BeO may not be practical in the near future due to anticipated environmental regulation.
Many thermal management methods for semiconductor applications are designed to dissipate heat primarily in the vertical, or "z" direction, underneath the heat generating device. For example, alumina substrates are often placed underneath the heat generating semiconductor chips so as to conduct and dissipate heat in the vertical direction away from the heat generating chip. Such designs are limited in their ability to dissipate heat laterally, i.e., in the "x" and "y" directions, because the thermal conductivity of an alumina substrate is low compared to metallic materials. Systems which are able to promote the lateral conduction and dissipation of heat have an advantage over systems which are primarily limited to conduction of heat only in the vertical direction. By conducting heat in the lateral directions, the cross-sectional area through which heat is conducted vertically is greater than the surface area directly under the heat dissipating device, such that a lower thermal resistance path is provided, resulting in an overall reduction of the device's operating temperature.
The use of ultra-thick thick films for dissipating heat is taught in U.S. patent application Ser. No. 08/038,379 to Myers et al. Whereas conventional thick films have a thickness in the range of about 13 to about 25 micrometers (about 0.0005 to about 0.001 inch), Myers et al. teach an ultra-thick thick film (UTTF) which can be formed to have a thickness of up to about 125 micrometers (about 0.005 inch). At such thicknesses, the ultra-thick thick film is highly suitable for use as a heat dissipating and current conducting system which is capable of conducting heat laterally (i.e., in the x and y directions) from a heat generating device.
In order to achieve such thicknesses, Myers et al. teach that the ultra-thick thick films are preferably composed of multiple layers of printed films. Each film layer is formed from an ink composition which includes about 80 to about 90 weight percent of a metal powder of copper, silver or other conductive material, with the balance of the composition being an inorganic binder and a screening agent. It is generally necessary to fire each layer separately in order to effect complete removal of the inorganic binder. The firing of relatively thick layers is generally impractical due to improper binder burnoff, which can have a detrimental effect on the solderability and/or adhesion strength of the film.
However, from the standpoint of processing and costs, it would be desirable if an ink composition were available that could form an ultra-thick thick film for conducting heat away from a semiconductor device, as taught by Myers et al., but would allow the film to be formed by a single printing, drying and firing operation. More particularly, such an ink composition would enable the formation of ultra-thick thick films having thicknesses of 125 micrometers and greater to be deposited as a single layer, which can then be fired during a single firing cycle to completely remove the binders in the ink composition and reliably yield a highly adherent film.